Filamentary cathode assembly



April 13, 1943. 1 GOODALE ErAL 2,316,415

FILAMENTARY CATHODE ASSEMBLY Filed March 3, 1942 sog Patented Apr. 13, 1943 FILAMENTARY CATHODE ASSEMBLY Lynn Goodale, Newark, and Lyle A. Backer, Denville, N. J., assignors to Federal Telephone and Radio Corporation, a corporation of Delaware Application March 3, 1942, Serial No. 433,170 1o claims. (cl. 25o-427.5)

This invention relates to filament construction and particularly to multistrand filaments used in vacuum tubes. Y

The object of the invention is to minimize deformations occurring in such filaments due to non-uniformity of expansion and contraction during heating and cooling of the filaments, in the operation of the vaccum tubes.

In high power vacum tubes such as well-known water cooled types, it has been customary to provide multistrand filaments, usually in a multiphase connection, to produce the high amount of electron emission required of such large tubes. Filaments have heretofore been connected in the well-known star relationship in which one end of each filament 4is connected to a neutral point and the other end to a source of heating voltage. Inasmuch as the neutral ends need not be connected to the power source, since no net current flows to or away from the neutral point, it has been the practice to locate the neutral point Within the tube and close to the filaments;` with the result that unneutral wire leads or supports from the lament have not been symmetrical with respect to the filament leads to the neutral point. It has been found that the unequal expansion and contraction under temperature changes of some of-the filament leads or rods relative to others .has caused an unsymmetrical sagging of the filaments so that they get out of line and even become short-circuited.

Applicants have avoided this diiiculty in accordance with this invention by a symmetrical arrangement of each of the filament strands and their connecting conductors, so that the changes in temperature produce substantially equal expansions and contractions on'either side of the points of symmetry. A related feature is theuse of cooling fins in connection with filament connectors of greater length than others to compensate for the effect of the greater length. As a result, the unequal expansion and contraction of the metallic parts is avoided and the filaments keep their shape.

The invention will be understood from the following detailed description and the accompanying drawing, of which Fig. l shows an elevation view, partially in section, of a filament arrangement in accordance with this invention.

Fig. 2 is an end view taken at line 2-2 of Fig. 1; f

Fig. 3 is a sectional view taken at line 3-3 of Fig. 1;

Fig. 4 is another sectional view taken at line 4-4 of Fig. l: and

Fig. 5 is another sectional view taken at line 5 5 of Fig. 1.

The drawing illustrates a multistrand filament, the particular arrangement shown being a sixphase star-connected filament supported from a vitreous re-entrant base portion I9 forming part of the envelope of a vacuum tube of the high power type. Only a portion of the tube envelope is shown, as the envelope and electrodes, other than the filament, constitute no part of this invention. The envelope will ordinarily be generally cylindrical in shape, part of this cylindrical portion being shown at II. At the annular reentrant portion I2, there are sealed through the glass, by long seals I3, a number of metallic supporting rods. Six of these supporting rods, namely I4, I5, I6, I1, I8 and I9 lead to respective ungrounded ends of six hairpin-shaped filaments 29, 2l, 22, 23, 24 and 25, the two filaments 2| and 22 lying directly back of filaments 25 and 24, respectively in Fig. 1.

Some additional rods, for example three,`26, 21 and 28 are likewise sealed through the annulus I2, and these three rods are connected with, and support a platform comprising metallic disc 29. A second metallic disc y3i) is supported in spaced relationship todisc 29 by rods 26, 2l and 28. These three rods are at neutral potential and will have connected to them the neutral terminals of the filaments. 'I'he six unneutral rods I4 to I9 inclusive, are passed through holes or openings in the neutral discs 29 and 39 so as not to touch these discs. Thus, rods I4 and I5 pass through opening 3l of disc 29, and through individual holes 32 and 33 of disc 39; rods I6 and I1 are passed through opening 34 in disc 29 and individual holes 35 and 36 of disc 39; and rods I9 and I9 are passed through opening 3l of disc 29, and individual holes 38 and 39 of disc 39. The circle along whichA the six un-neutral rods I4 to I 9 and the three neutral rods 26 to 28 are spaced, preferably uniformly, at the glass seal is of larger diameter than the circle on which these same rods are located where they pass through discs 29 and 39; so these rods are bent to fiare outwardly from just below disc 29 as indicated at Ma, I5a, Ita, I la, 26a and 21a; the corresponding flares for the other rods not being shown. The circle along which all nine of these rods are spaced just above vthe seals I3, are preferably bound together for increased rigidity by a glass ring member formed by individual glass seals 49 tied each to the next by a sturdy web of glass 4I There is fastened to disc 29, a set of six rods 42, 43, 44, 45, 46 and 41. 'Ihese rods terminating at disc 29, as shown in Fig. 5, extend parallel to the rods I4 to I9, through respective holes 48, 49, 50, 5I, 52 and 53 in disc 39, without touching this latter disc. All twelve rods I4 to I9, and 42 to 41 extending through disc 30 to the filaments are preferably equally spaced around a circle as shown in Figs. 3 and 4, and the upper ends of rods 42 to 41 connect with the ends of the respective hairpin filaments other than the ends to which rods I4 to I9 are connected.

Each of the six hairpin-shaped filaments is formed with a loop in the middle, providing a pair of substantially parallel arms of equal length. For the purpose of supporting the six hairpin filaments, there is provided a centrally located shaft 54 fastened at the middle of the platform 29 to which it may be attached as by welding, and closely fitted through the center of disc 3D. This assembly of the platform 29 and disc 3B and shaft 54 is supported by rods 25, 21 and 28. The opposite end of shaft 54 supports at its end a disc 55 which has around its periphery, holes through which are placed respective insulating bushings 55, 51, 58, 59, 60 and 6I. There is passed through each bushing a respective wire loop 62, 63, 54, 65, 66 and 61, the protruding ends of which are turned over as shown in Fig. 2 to hold each loop in its bushing. A positioning piece 58 may be placed over the top of the disc 55 and fastened as by screws 69, this positioning piece providing slotted tongues which engage with corresponding grooves in the insulating bushings to keep them from turning.

The six hairpin-shaped laments are looped through their respective supporting loops as shown, and the ends of the hairpins fastened to the respective rods I4`to I9 and 42 to 41 in a suitable manner, such as by thin metal sheaths 1I wrapped around the overlapping ends of the filament and rod, and fastened as by welding.

A cooling fin 12 is attached to each of rods I4 to I9 inclusive, to carry heat away from it.

B'y reason of this construction, the supporting platform 29, together with discs and 55, the central shaft 54 and the rods 25, 21 and 28 are placed at neutral potential. The six lament connecting rods 42 to 41 are likewise at neutral potential, so that one end of each filament is neutral and the other end of each filament is unneutral for connection with a suitable source of filament voltage. This is the well-known sixphase star-connection. The source of filament heating voltage will be connected to the rods I4 to I9 protruding from the glass envelope; andthe neutral potential rods 2S, 21 and 28 may likewise be connected to ground outside the tube if desired.

In the operation of the tube, great heat' is developed at the filaments and much of the heat is carried through. theY filament connecting rods I4 to I9, and 42 to 41. When the filaments are heated and cooled, there is a corresponding expansion and contraction of the filament strands and of these connecting rods. By reason cf the fact that the substantially parallel sides of each filament strand are of equal length, and that the parallel filament connecting rods I4 to I9, and 42 to 41 extend for a substantial length, the expansion and contraction of the strand and connecting rod on each side of each filament loop is nearly the same so that unequal sagging does not occur; and after the filaments cool, they become restored to substantially their original position.

Owing to the fact that the un-neutral rods I4 to I9 have some additional length through bends I 4a. to I9a, beyond the neutral connecting rods 42 to 41, there is some tendency for the rods I4 to I9 to expand, when heated, somewhat more than rods 42 to 41. This is compensated for by the cooling fins 12 placed on rods I4 to I9 just above platform 29. The fins are proportioned so that they carry away from rods I4 to I9 suliicient of the heat sent through them by the filament to cool them enough so that they do not expand and contract in the direction of the filament any more than rods 42 to 4i'.

What is claimed is:

1. A multistrand filament assembly comprising a, plurality of filaments, each of which is a wire looped at the middle, individual supports on which the loops are supported, individual rods to which opposite ends of each filament are attached, a supporting platform to which one rod from each filament is fastened, the other rod from each filament extending past and out of contact with said platform and away from the region of highest temperature.

2. A multiphase filament assembly comprising a plurality of wire filaments looped at the middle and having the portions on either side of the middle loop extending nearly parallel, individual supports from which the middle loops are suspended, a plurality of substantially parallel rods, the individual ends of each filament being attached to an individual one of the rods, a supporting platform in a plane substantially perpendicular to the plane of the rods, the rod from one end of each filament being attached to said platform, the other rod from each filament extending as far as said platform but out of contact with said plate and being bent substantially at said platform to continue in a different direction, the last-mentioned rods being terminals of the multiphase filament.

3. A multistrand filament assembly comprising a centrally supported metallic shaft, a supporting means including a disc-shaped platform attached to said shaft, a plurality of filaments, each of which is in the form of a wire looped at the middle and arranged with an equal length of substantially parallel lengths on either side of the loop, each loop being supported from au individual support attached to said supporting means and insulated from each other and from the shaft, said parallel filament sides extending generally parallel to said shaft, individual terminal rods attached toV opposite ends of each filament and extending in the same general direction parallel to said shaft, one rod from each filament being attached to said platform, the other rod from each filament extending through but insulated from said platform.

4. An assembly according to claim 3 in which said other rods are bent to take a different direction after passing through said platform.

5. A multiphase filament assembly comprising a plurality of filaments, conducting leads to each end of each filament, one lead to each lament being connected to a point common to all filaments and the other lead from each filament being connected to a source of polyphase heatingr voltage, each filament being symmetrically arranged with respect to its midpoint, and the conducting lead from the common point to each filament being of effectively equal length to the other conducting lead to each filament.

6. A multistrand filament assembly comprising a plurality of filaments, each of Which is looped at its middle and has the portions on either side of the loop extending substantially parallel and of equal length, a pair of conducting leads connecting to the respective ends of each filament, one lead of each pair connecting to a common point and the other lead of each pair connecting 4to a source of heating voltage, said other leads extending substantially parallel to the leads connected to the common point for the greater part of the length of said leads.

7. A multistrand filament assembly comprising a plurality of filaments, each of which is looped at its middle and has the portions on either side of the loop extending substantially parallel and of equal length, a pair of conducting leads connecting to the respective ends of each filament, one lead of each pair connecting to a common point and the other lead of each pair being connected to a source of heating voltage, said other lead extending substantially parallel to the lead connected to the common point as far as said common point, said other lead being bent out of line from said parallel direction adjacent said common point of connections.

8. A multistrand filament assembly comprising a plurality of filaments, each of which is looped at the middle to form a hairpin-like shape with the portions of the filament on either side of the loop extending substantially parallel to each other and of equal length, means spaced around a circle for supporting each filament at its loop, individual rods connected with opposite ends of each filament and extending in the general direction of said filament to which it is connected, said rods being spaced around a circle and being substantially parallel with each other, a metallic connecting means for acting as a common potential point, there being one of the rods from each filament connected to said metallic connecting means, the other rod from each filament being bent out of said parallel direction at a place adjacent said connecting means and connected therebeyond to a source of filament heating voltage.

9. A multistrand filament assembly comprising a plurality of filaments, each of which is looped at the middle and has the portions on either side of the loop extending substantially parallel and of equal length, a pair of conducting leads connected to the respective ends of each filament, a first lead of each pair connecting to a common point and the second lead of each pair extending substantially parallel to and past the end of the first lead, and a cooling fin connected to the second lead andproportioned to conduct enough heat away from the second lead so that said second lead expands and contracts relative to the filament to the same extentas the first lead.

10. A filament assembly comprising a filament looped at its middle and having portions on each side of the loop extending substantially parallel and of equal length, an individual conducting lead connected to each end of the filament, said leads extending substantially parallel but of unequal length, and a cooling fin connected to the longer one of said leads, said cooling fin being proportioned to conduct away enough heat from the longer heat so that both leads expand and contract relative to the filament under temperature changes, to substantially the same extent.

LYNN GOODALE. LYLE A. BACKER.. 

